This invention pertains generally to III-V semiconductor devices and more particularly to III-V transmission photocathodes and processes for making the same.
Transmission mode GaAs photocathodes generally comprise an active GaAs layer on a transparent substrate. For maximum efficiency, the active layer should be thin (2 microns) and of high quality. In order to minimize the defect density at the interface of the active layer and substrate, the active layer and substrate should be closely matched in lattice constant. Heretofore, GaAs transmission photocathodes have been prepared by vapor phase epitaxy on transparent substrates such as Al.sub.2 O.sub.3, MgAl.sub.2 O.sub.4 and GaP. The lattice mismatch between the GaAs active layer and the substrates makes such devices very inefficient. It has been found that a GaAs-AlGaAs heterojunction prepared by liquid phase epitaxy is both transparent to visible radiation and has a low defect density. In the past, GaAs-AlGaAs photocathode structures have been formed on temporary substrates, and the temporary substrates have been removed by techniques such as mechanical and chemical polishing. These prior art techniques are primarily applicable to devices with small surface areas without critical surface requirements, and they generally do not produce satisfactory results in photocathodes.
Mechanical polishing is generally done with progressively smaller grits, and the smaller grits generally do not remove the initial surface scratches produced by the larger grits. The surface damage diffuses light in optical applications and inhibits epitaxial growth of additional layers. In transmission photocathode applications, surface blemishes are particularly undesirable because the irregularities can trap electrons, causing dark spots in the image, and they can also create emission points, causing light spots in the image.
In the past, there have also been some attempts to prepare the surfaces of III-V devices by etching away a portion of a layer. However, attempting to remove substantial material from a layer by etching generally results in the removal of too much material at the edges and the rounding of the surface. Moreover, accurate thickness control is difficult with etching, and attempts to etch thin layers can result in inadvertent removal of an entire layer.